A Choice of Blue

A Choice of Blue

  • By Susan Martin
  • /
  • August 2018 - Vol. 4 No. 8
  • /

This article will focus on two blue-flowering plants from the Asteraceae family that are similar in appearance, but behave very differently. Both add great, long-lasting color from mid-summer through fall. Both are deer resistant. One is a nonnative annual; one is a native perennial. The perennial tends to spread very aggressively through both rhizomes and self-seeding. With that caveat in mind, you can evaluate each plant for use in your landscape.


Ageratum is a genus of about 50 species of flowering annuals and perennials from the large and varied plant family Asteraceae (also called Compositeae). The genus name presumably comes from the Greek “a geras” meaning “not old age” because the flowers hold their color for such a long time.


Commonly called “floss flower,” A. houstonianum is an annual plant native to Mexico and Central America. It was collected by William Houston, a Scottish surgeon and botanist (hence, houstonianum), in the early 1700s. Cultivation in Europe led to the establishment of A. houstonianum as a garden ornamental in Europe and the United States by the 19th century. Seed companies successfully introduced F1 hybrid cultivars in the mid-20th century.

A. houstonianum is easily grown in average soil in full sun, though it prefers rich soil with good drainage and consistent moisture throughout the growing season. In hotter climates, light afternoon shade is appreciated. Plants tend to wilt quickly if soils are allowed to dry out. Frost-tender, it is usually removed from the garden after the first frost.

Ageratum houstonianum, Floss Flower

The plant features fluffy flowers in flattened-to-slightly-rounded clusters. “Floss flower” refers to the thread-like appearance of the blooms. Each flower cluster consists of 5 to 15 tubular florets. Species flowers are medium blue, but cultivars have been developed in a range of colors including white, pink, mauve, red, and bicolor. Flowers fall to the ground when spent and are replaced by new blooms. Although not necessary, deadheading helps maintain a neater appearance. Leaves are typically oval to heart-shaped, hairy, slightly quilted, and soft green; lower leaves are opposite, upper leaves are alternate.


Blue and white A. houstonianum with pollinators

Bees and butterflies are attracted to the fragrant flowers; A. houstonianum is listed as one of the nectar sources for the butterfly Eumaeus atala, which is listed as rare and vulnerable in Florida. Birds, especially goldfinches and juncos, eat the plant’s seeds.


A very useful characteristic of A. houstonianum is that it comes in a range of sizes, from 6” to 30” tall. The shorter varieties are commonly used as bedding plants in the front of the garden, along walkways, and in containers. Plants in the 12” to 18” range can be used further back in the garden. ‘Blue Horizon’ is a more erect cultivar that can grow to 30” tall. Its beautiful lavender-blue flower is a welcome, deer-resistant addition to the center or rear area of the perennial bed. Tall cultivars are an excellent choice for gardeners who enjoy cut flowers; the blooms are frequently used as dried flowers.


Most of the A. houstonianum cultivars are propagated from seed, and are predominantly F1 hybrids. F1 hybrid refers to the selective breeding of a plant by cross-pollinating two stable seed lines (called inbred lines). In genetics, the term F1 is an abbreviation for Filial 1 – literally “first children.” Seed produced by F1 plants is genetically unstable and should not be saved for use in following years. The plants will not be true-to-type, and they will be considerably less vigorous.

Some of these cultivars are fragrant; some, reportedly, can cause skin irritation when handled. The latter seems to contradict the use of ageratum as a folk remedy for healing wounds. The information I found was not always consistent, but I have indicated the characteristics of fragrance and possible skin irritation when these were specifically noted in cultivar descriptions.

  • ‘Bavaria’ is 12-18” tall; white near the center turning to blue near the edge.
  • ‘Blue Blazer’ is 5-6” tall; first commercial F1 A. houstonianum hybrid; better plant uniformity and vigor; blooms earlier than open-pollinated cultivars.
  • ‘Blue Danube’ is 6-8” tall; mid-blue; one of the best cultivars for uniformity, earliness of bloom, and general performance; may irritate skin.
  • ‘Blue Fields’ is 6-12” tall; blue-violet.
  • ‘Blue Horizon’ is 24-30” tall; medium blue-purple; fragrant; a great cut flower; does not set seed.
  • ‘Blue Mink’ is 6-12” tall; powder blue; open-pollinated cultivar.
  • The F1 hybrid Hawaii Series offers ‘Hawaii Blue’, ‘Hawaii Pink’, and ‘Hawaii White’. Compact, bushy plants are 6-12” tall, thrive in sun and in partial shade, bloom earlier in the summer, and last longer into the fall.
  • ‘Pinky’ is 8” tall; compact bushy plant; salmon-pink flowers.
  • ‘Pinky Improved’ is 6-9” tall; compact, showy plant; flowers are dark pink in the centers, fading to pale pink at their fringed edges.
  • ‘Purple Fields,’ an F1 cultivar, is 6-12” tall and 12” across; unusual blue-purple flowers.
  • ‘Southern Cross’ is 6-12” tall; white centers turning to cornflower-blue toward the edge; suited for containers in part-shade; handling plant may cause skin irritation or allergic reaction; flowers are sterile; does not set seed; good cut flower.
  • ‘Summer Snow’ is 6” tall; F1 hybrid with fluffy white flowers
  • ‘Trinidad’ is 6” tall; unique, early-blooming blend of small white, blue, violet, and pink flowers.


A. houstonianum ‘Blue Horizon’ and Gomphrena ‘Ping Pong Purple’

The soft blue of low-growing A. houstonianum pairs well with pink-flowering plants. White, yellow, orange, and red flowers also provide nice contrast. Check plant selections for light requirements. For example, if pairing with pink wax begonias (Begonia x semperflorenscultorum), look for A. houstonianum cultivars that do well part-sun, such as the Hawaii Series. In sunny conditions, pair A. houstonianum cultivars with yellow marigolds, white or pink petunias, or dwarf Shasta daisies (Leucanthemum x superbum). Taller A. houstonianum cultivars work well with taller Shasta daisies or with Gomphrena; look for the Gomphrena ‘Ping Pong’ series in white, lavender, or purple.


When grown in good soil with adequate light, water, and drainage, A. houstonianum is typically a very dependable ornamental. It is also quite resistant to damage from Japanese beetles; watch for aphids, whiteflies, and red spider mites. Powdery mildew is an occasional problem, particularly in hot and humid climates where soils are kept on the dry side and air circulation is poor. Avoid watering from overhead if powdery mildew is an issue. Root rot may occur in poorly drained soils.


For F1 cultivars, gardeners should purchase seed each growing season. Seed may be started indoors in late winter (about 8-10 weeks before last frost). Surface sow, barely covering with vermiculite or just gently tamping down the potting mix. Exposing the seeds to light helps germination. Germination usually takes 7-21 days. Seed may also be sown directly in the garden after the last-frost date; however, the flowering season will be shorter (late summer to frost). Seeds are tiny and difficult to handle, particularly for sowing outdoors. Plant seedlings 6-8″ apart in a sunny spot after threat of frost has passed.

Some gardeners prefer buying flats of A. houstonianum seedlings, rather than starting from seed. Just check the labels to make sure you are getting plants of the desired color and height.


Studies have shown that essential oils and extracts from the leaves of A. houstonianum exhibit antifungal, antimicrobial, and acaricidal (pesticides that kill members of the arachnid subclass Acari, which includes ticks and mites). The leaf oil is toxic to the tick Rhipicephalus lunulatus (Pamo et al., 2005). Leaf oils also exhibit mosquitocidal activity, as well as repellency against mosquitoes.

The leaf extract has been used against Meloidogyne hapla, a nematode that causes significant damage and crop losses in temperate zones (Thoden et al., 2009). Chemicals that induce the premature metamorphosis and sterilization of some insect species were isolated from A. houstonianum and are marketed as Precocene I and Precocene II (Jacobson, 1982). A Chinese study in 2014 looked at the effectiveness of the essential oils of A. houstonianum’s leaves as an insecticide against booklice, and potentially as an insecticide against grain storage insects. Fungicidal activity against Phytophthora infestans has reduced the severity of that disease in tomato crops (Goufo et al., 2010).

The plant species is used in traditional medicine to treat skin infections and sore throats; the leaves are applied to wounds to stop bleeding. A study by Tennyson et al. (2012a) reports that the species has a high antioxidant activity with potential cosmetic and medicinal uses.

A. houstonianum can be toxic to grazing animals and cause liver lesions.


A. houstonianum has escaped cultivation and naturalized in many temperate regions of the world, being declared an invasive in many areas including: China, Taiwan, parts of Africa (particularly South Africa) Australia, New Zealand, Peru, Tahiti, Fiji , French Polynesia, Cuba, and Hawaii.


Conoclinium coelestinum, synonymous with Eupatorium coelestinum, commonly called blue mistflower or blue boneset, is an herbaceous perennial native to the Eastern United States. Also in the Asteraceae family, it is a species similar to annual ageratum. Both are in the same tribe as bonesets, thoroughworts, and snakeroot. It looks like annual ageratum and in that regard is sometimes commonly called hardy ageratum. Frequently described as a late-summer-to-fall-blooming perennial, it often starts blooming by mid-summer in Zone 7, and then flowers well into fall.


Conoclinium coelestinum, Blue mistflower

Blue mistflower can grow 1-3’ tall. Each flat, irregular flower head consists of 30-70 five-petaled disk flowers whose long stamens cause the fuzzy appearance of the flower. There are no ray flowers on this member of the aster family. The color has been described as clear blue, powder blue, azure blue, bluish-purple, reddish-purple, blue-pinkish, and pink-purplish. No matter the description of its hue, it’s beautiful color is a welcome addition to the summer garden.

Conoclinum coelinium, Blue mistflower

Blue mistflower grows in full sun to partial shade as long as it has plenty of moisture. Although it will survive in dry soils without supplemental watering, plants will be shorter, flowering will not be as grandiose, and it will not spread as much as plants that get regular irrigation. It can thrive in moist loam, sandy, or clay soils.

This perennial native is an aggressive spreader through both self-seeding and creeping rhizomes. It produces abundant seed that is wind-carried, often showing up in places where it may not be welcomed. It covers the ground with opposite, wrinkled, coarsely-toothed, triangular leaves that are 1-3” long on short petioles (stalk that attaches the leaf blade to the stem). Although fairly easy to pull, the plants can stray far from where they were originally intended.



Blue mistflower is hardy in USDA Zones 4-11. It is found in floodplains, along pond and stream margins, in fields and wet meadows, and along road shoulders from New Jersey, west to Wisconsin and Kansas, and south to Texas and Florida.


Because of its spreading characteristics, blue mistflower is a great addition to an open meadow area, or as a border along a woodland. It can, however, become an unruly, domineering addition to a formal perennial garden. Within a suitable setting, however, its sweep of color is a real WOW!


The Fall 2015 issue of HabiChat from the Maryland Department of Natural Resources describes the benefits to pollinators:

Blue mistflowers are a late-season butterfly favorite, attracting monarchs, sulphurs, buckeyes, and more. Long- and short-tongued bees, flower flies, moths, and beetles all can be found nectaring. Caterpillars, such as the clymene moth and lined ruby tiger moth, will dine on its foliage. Few mammalian herbivores will take a bite, however, as the leaves are bitter to the taste.


Sow seeds in autumn or provide cold stratification if planted in spring (place seeds in a plastic bag with moistened sand or a moistened paper towel, seal, and keep in the refrigerator for about 3 months). Propagate by root division in spring or when early plants appear.


Each of these plants provides a beautiful blue to the garden when many other summer blooms are fading. Both are long lasting, resistant to deer, free from major pest problems, and attract pollinators. The perennial, blue mistflower, is an aggressive spreader, and its location should be thoughtfully selected.


Ageratum Houstonianum, http://www.plantfileonline.net/plants/plant_details/9

Ageratum L. ‘John Eustice’: A New Vigorous Lavender–Blue Flowered Summer Annual,” http://hortsci.ashspublications.org/content/49/4/509.full

Plants That Attract Wildlife, Pollinators in Urban Landscapes, http://msue.anr.msu.edu/resources/how_to_protect_and_increase_pollinators_in_your_landscape/better_habitat_for_bees

Conoclinium coelestinum, http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=j870

Ageratum houstonianum ‘Blue Horizon,’ http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=260781&isprofile=0&

Ageratum, Ageratum houstonianum,” https://wimastergardener.org/article/ageratum/

The Plant List, http://www.theplantlist.org/1.1/browse/A/Compositae/Ageratum/

Cornell Growing Guide: Ageratum, http://www.gardening.cornell.edu/homegardening/scene8ada.html

Ageratum Varieties, https://davesgarden.com/guides/pf/go/84590/

Wisconsin Horticulture: Ageratum, https://hort.uwex.edu/articles/ageratum,

Ageratum, University of Vermont, http://pss.uvm.edu/pss123/annagera.html

“Wildflowers of the United States,” https://uswildflowers.com/detail.php?SName=Conoclinium%20coelestinum

Florida Plant Encyclopedia, http://floridata.com/Plants/Asteraceae/Conoclinium+coelestinum/838

“Protecting and Enhancing Pollinators in Urban Landscapes for the U.S. N. Central Region,” http://msue.anr.msu.edu/resources/how_to_protect_and_increase_pollinators_in_your_landscape/better_habitat_for_bees

Conoclinium coelestinum, Lady Bird Johnson Wildflower Center, https://www.wildflower.org/plants/result.php?id_plant=coco13 

Conoclinium coelestinum, Missouri Dept. of Conservation, https://nature.mdc.mo.gov/discover-nature/field-guide/mist-flower-wild-ageratum-blue-boneset

“Maryland Native Plant Profile: Blue Mistflower,” http://dnr.maryland.gov/wildlife/HabichatArchive/Habichat48.pdf

“Isolation of Insecticidal Constituents from the Essential Oil of Ageratum houstonianum Mill. against Liposcelis bostrychophila Badonnel,” Journal of Chemistry, https://www.hindawi.com/journals/jchem/2014/645687/

Invasive Species Compendium, https://www.cabi.org/isc/datasheet/3573

“Gomphrena–An Antidote for the Late Summer Blahs,” http://piedmontmastergardeners.org/article/gomphrena-an-antidote-for-the-late-summer-garden-blahs/





Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.